1. Field of the Invention
The present invention relates to a method of control, for use in an injection molding apparatus utilizing a motor, particularly an injection molding apparatus in which linear motion and rotary motion of a screw is conducted by a motor, for controlling the switching of a transmission of the driving force of said motor according to a detection signal from a sensor.
2. Related Background Art
In the injection molding method for producing molded articles by injecting a molding material such as plastics into a mold, there have principally been employed hydraulic and electric injection molding apparatus.
In the following there will be explained a conventional hydraulic injection molding apparatus with reference to FIG. 5. Said apparatus is principally composed of a hopper 71; a screw 73 for blending and measuring the resin, supplied from said hopper 71, in a cylinder 72; a motor 51 for rotating said screw 73; and straight-moving cylinder 52 for injecting thus blended resin. The motor 51 and straight-moving cylinder 52 are usually hydraulic for the ease of obtaining a high output. Then, in the following there will be explained an electric injection molding apparatus disclosed in the Japanese Patent Publication Sho 61-57168, shown in FIG. 6. A screw 53 housed in a heating cylinder 66 is fixed to a screw rotating gear 54 positioned in the back. A support member 56, slidably supported by a guide bar 57, rotatably supports said screw rotating gear 54 and is provided, at the front end thereof, with a ball nut 55 in which is fitted a ball bearing screw 58 maintained in contact with the shaft of the gear 54. On said ball bearing screw 58 there is fixed a rotating gear 59. Said screw rotating gear 54 and said rotating gear 59 mesh respectively with driving gears 63, 64 provided on the rotary shaft of a motor 62 and connected respectively thereto by clutches 60, 61.
Behind said rotating gear 59, there is provided a back pressing brake unit 65 for receiving the retracting movement of the screw 53 from behind the gear 59. When the screw 53 is retracted by the blending and measuring of the resin from the hopper 74 into the heating cylinder 66, the ball nut 55 is retracted through the gear 54 and the support member 56 to rotate the ball bearing screw 58, whereby the gear 59 is rotated. By pressing the end face of said rotating gear 59 with the brake unit 65, the rotation of said gear 59 can be controlled by the slip torque therebetween, and back pressing to the screw 53 can be achieved in this manner.
However the conventional injection molding apparatus explained above have been associated with various drawbacks as will be explained in the following.
In the hydraulic injection molding apparatus as shown in FIG. 5;
(1) A large space is required for installation, because of peripheral equipment such as hydraulic pump and pipings; and PA1 (2) It is difficult to operate the molding apparatus in a clean condition, because of oil mists etc. generated by the hydraulically driven equipment. PA1 (1) Since the back pressure has to be controlled by the sum of a force for converting the linear motion for example of the ball nut at the screw retraction into a rotary motion for example of the ball bearing screw, and a force generated by the slip torque of a plate pressing the end face of the gear, there are required complex settings of multiple parameters such as rotary resistance for example of ball bearing screw, frictional force of the braking plate, output generated by the brake etc.; and PA1 (2) Since the rotated gear and the propelled ball nut are supported by the same support member and are constructed to integrally move with the screw, there is required a guide member for stopping the rotation of the support member, whereby a large space is required around the driving system, and the structure becomes inevitably complicated.
On the other hand, the electric injection molding apparatus as shown in FIG. 6 is no longer associated with the above-mentioned drawbacks, but is still associated with following drawbacks:
In consideration of the foregoing, the present inventors have designed an electric injection molding apparatus for effecting the rotary motion and linear motion of a screw housed in a heating cylinder by means of an electric motor, comprising a rotating mechanism, a linear moving mechanism and a back pressurizing mechanism behind said screw in the above-mentioned order or in an order of the linear moving mechanism, rotating mechanism and back pressurizing mechanism, wherein said rotating mechanism comprises a rotary shaft for rotating said screw and a first rotary driving force transmission mechanism for transmitting the rotary driving force of said motor to said rotary shaft; said linear moving mechanism comprises a guide shaft having stopping portions at the ends thereof, a hollow ball bearing screw slidably fitted on said guide shaft and stopped by said stopping portions, a ball nut engaging with said hollow ball bearing screw, and a second rotary driving force transmitting mechanism for transmitting the rotary driving force of said motor to said ball nut; and said back pressurizing mechanism comprises a back pressure cylinder, and a cylinder rod for pressing said guide shaft or said rotary shaft.
Such apparatus allows control of the back pressure with a simple mechanism which is easily adjustable, and also allows compactization of the screw propelling and rotating mechanisms.
In such electric injection molding apparatus, by feeding the molding material such as resin into the heating cylinder under the rotation of the screw, the material is blended in the cylinder while the screw is retracted, whereby the blended material is accumulated in front of the screw.